The present invention relates to methods and arrangements for diversity selection for antenna paths.
In most digital communications systems different kinds of diversity is used to reduce the effects of fading. Antenna diversity improves the receiver sensitivity and is one of the most common schemes, normally implemented as dual antenna diversity at the receiver.
There is a number of schemes for combining multiple antenna signals received with diversity. In post-detecting schemes, the combining can be done completely in the digital domain. Coherent combining schemes such as equal gain combining, maximal ratio combining and interference rejection combining are efficient post detecting schemes. The problem with post detecting schemes is that it requires multiple receivers. In a mobile station, the receiver complexity needs to be kept at a minimum. Pre-detection schemes are therefore desirable since only one complete receiver chain is required.
In the International Patent Application WO 95/11552, a diversity receiver having two receiver branches is shown. A control signal based on comparison between strength of the received signals is generated. Switching means are arranged to change-over a signal from either one of the receiver branches to a receiver output in dependence of the control signal. This previous known configuration causes however problem when implemented in a mobile station. Multiple antennas implemented in a mobile station are often a regular dipole antenna combined with an internal antenna. The internal antenna then gives an attenuated signal causing the weaker antenna to be selected less than 50% of the time. The implication is a reduced diversity gain compared to balanced antenna paths.
Problem with previous known techniques arises when antenna diversity is used in arrangements with both internal and external antennas. The internal antenna causes reduced diversity gain due to an attenuated signal caused by framing of the surrounded body.
This problem is solved in accordance with the invention by estimating average interference signal powers over a period of time for each of the received signals. When using these estimates to offset the selection criteria by the difference in interference powers, the antennas are selected in dependence of the best received carrier signal in relation to the average interference signal strength.
In one embodiment of the invention the interference strength of received radio signals is accumulated by adding the interference strength of a radio signal received by the selected antenna to previous accumulated values. The average interference signal strength is thereafter generated for the accumulated values of the signals received from each one of the antennas. A threshold offset signal corresponding to the difference in strength between the generated average interference signals is then generated. A selection signal corresponding to the threshold offset signal subtracted from the difference in strength between the carrier of received radio signals is thereafter generated. One of the antennas is then selected in dependence of the value of the selection signal. As a result, the signal having the best received carrier signal in relation to the average interference signal strength is received.
In another embodiment of the invention the interference strength of a plurality of received radio signals is accumulated by adding the interference strength of a radio signal received by the selected antenna to previous accumulated values. The average interference signal strength is thereafter generated for signals received from each one of the antennas. A signal corresponding to carrier in relation to average interference signal strength is thereafter generated and forwarded to a selector which points out the signal having the best carrier in relation to interference. The antenna delivering this signal is selected.
One object of the present invention is to obtain a diversity gain for unbalanced antenna paths equal to the diversity gain for balanced paths.
One of the advantages with the present invention is thereby that the diversity gain is the same for unbalanced antenna paths as for balanced antenna paths.
The invention will now be described more in detail with reference to exemplifying embodiments and also with reference to the accompanying drawings